Download Drift demand on facades - Australian Earthquake Engineering Society

AEES 2009 Annual Conference
Seismic Assessment of Glazed Façade Systems
Sivanerupan S1, Wilson JL2, Gad EF3, Lam NTK4
1.Corresponding Author. PhD student, Faculty of Engineering, and Industrial
Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia.
Email: [email protected]
2.Professor of Civil Engineering, Faculty of Engineering and Industrial Sciences,
Swinburne University of Technology, Hawthorn, VIC 3122, Australia. Email :
[email protected]
3.Associate Professor, Dept. of Civil and Environmental Engineering, The University
of Melbourne, Parkville, VIC 3010, Australia. Associate Professor of Civil
Engineering, Faculty of Engineering and Industrial Sciences, Swinburne University
of Technology, Hawthorn, VIC 3122, Australia Email:
[email protected]
4.Professor, Dept. of Civil and Environmental Engineering, The University of
Melbourne, Parkville, VIC 3010, Australia. Email: [email protected]
Abstract
The curtain wall façade system is popular in all types of buildings including commercial,
industrial and institutional structures. The structural design of curtain walls normally takes
into account in-plane and out-of-plane loading from wind, thermal movement and
deflection from supporting structural elements due to gravity loads and creep. Seismic
loads on the structure can potentially impose significant in-plane loading on the glazing
system and may lead to damage if adequate detailing is not provided. In this paper,
codified inter-storey drift limits for buildings are reviewed and seismic drift assessment
methods of glazed façades in buildings are suggested with increasing accuracy and
complexity. Performance of glass façade systems are then assessed with analysis results
and conclusions presented.
Key words: curtain walls; inter-storey drift; seismic performance.
1. INTRODUCTION
Glazed façade systems may be subject to racking action due to the relative lateral
movement of building from earthquake excitation. The performance of façade system is
dependent on the amount of drift and the interaction of the glass panel with the façade
support structures. There are two major concerns related to architectural glazing
performance during and immediately after a seismic event (Saflex Solutia Architectural
Glazing 2007):

Hazards to people from falling glass. This may cause injuries at street level from
broken storefront and elevated glazed panels.

Building down time and cost to repair. Bringing a building back to operation can
be delayed by a breached building envelope due to glazed façade systems damage.
It is evident that in the past earthquakes, glazed systems with sufficient clearance between
edges of the glass panel and the supporting structures have performed well. The
performance of fixed windows and storefront glazing systems has been tested in
laboratories over the past few decades. Researchers have suggested improvements such as
addition of smooth corners around each glass panel and adoption of more robust glass
types such as heat strengthened, toughened and laminated glasses (Behr 2006). A limited
number of analytical studies related to the simulated seismic performance of glazed façade
systems were also developed (Memari et al. 2007).
The Standard for earthquake actions in Australia, AS 1170.4 (2007), limits the inter-storey
drift to 1.5% in buildings and states that, the “attachment of cladding and façade panels to
the seismic-force-resisting system shall have sufficient deformation and rotational
capacity”. However the seismic drift performance of glazed façades is generally not
considered in the design stage by façade engineers.
Analysis results indicate that the inter-storey drift is much less than 1.5% for most
buildings in Australia for the 500 year return period (RP) event except for soft storey
structures. However a simplified approach is required to calculate the maximum in-plane
drift demand and assess the performance of façades. A detail approach is not considered
practical since façade engineers are given limited structural information on the building.
This paper addresses three key issues: (i) a review of codified inter-storey drift limits and
industry practice (ii) assessment methods for calculating inter-storey drift demand, and
(iii) in-plane drift capacity of glazed façade systems. The outcome of this study will be the
development of a simple assessment procedure to ensure a minimum level of protection
against seismically induced damage in glazed façades for new buildings as well as for
existing buildings for retrofitting, for regions of low to moderate seismicity such as
Australia.
2. CODIFIED INTER-STOREY DRIFT LIMITS AND INDUSTRY PRACTICE
Drift provisions in Standards are recommended for serviceability and ultimate limit states.
“Structural design actions”, AS/NZS 1170.0 (2002) provides out-of-plane and in-plane
serviceability limit state criteria for building elements. The Standard recommends an inplane drift limit of H/600 for the brittle masonry wall (where H is the height of the wall)
but no limits are specified for glazed façade systems.
The Australian Standard “Concrete structures”, AS 3600 (2001) specifies in clause 2.4.3,
“unbraced frames and multi-storey buildings subject to lateral loading shall be designed to
limit calculated inter-storey lateral drift to H/500 of the storey height”. This is aimed for
the serviceability limit state of the building mainly for wind loading. Whilst the Standard
for “Steel structures” AS 4100 (1998) recommends compliance with AS 1170.4 (2007).
AS 1170.4 (2007), clauses 5.4.4 and 5.5.4, specify that, “the inter-storey drift at the
ultimate limit state, calculated from the forces determined according to strength and
stability provisions shall not exceed 1.5% of the storey height for each level and “the
attachment of cladding and façade panels to the seismic-force-resisting system shall have
sufficient deformation and rotational capacity”. This requirement is for the ultimate limit
state of the building for seismic performance and a 3600 mm height floor is equivalent to a
relative building deflection of 54 mm.
The New Zealand Standard “Earthquake actions”, NZS 1170.5 (2004) specifies in clause
7.5 that, “a maximum inter-storey drift limit of 2.5 % is applicable for the ultimate limit
state of 500 year RP event. In the case of a 2500 year RP near fault event, this limit has to
be increased to 3.75%. Drift limits of 2.5% and 3.75% create demands of 90 mm and 135
mm respectively on façade systems, assuming a storey height of 3600 mm.
The Council on Tall Buildings, Group SB (1979), examined the serviceability wind drift
criteria from industry and literature and found that drift limits ranging from 0.001H to
0.004H were used. However the Council states that buildings designed in the past have
been known to perform satisfactorily when designed for drift limits from 0.002H to
0.005H. ASCE Task Committee found that most of the design for institutional,
commercial, and residential building types used drift ratio in the order of 0.002H to
0.0025H for steel framed buildings.
The Australian Standard “Glass in buildings-Selection and installation” AS 1288 (2006),
provides guidance for the strength and serviceability design of glass subject to out-ofplane wind loading but does not comment on in-plane effects. From discussions with
industry experts, the glazed façades are also designed for in-plane racking performance
due to wind loading which is usually H/500 for serviceability conditions. Methods to
assess the drift demand on buildings for the purpose of assessing glazed façades are
described in section 3.
3. ASSESSMENT METHODS FOR CALCULATING INTER-STOREY DRIFT
DEMANDS
Seismic drift demand on buildings can be investigated in many ways using elastic or
inelastic approaches with static or dynamic analyses. Seismic drift assessment methods in
regular buildings are suggested using five tiers with increasing accuracy and complexity.
Tier # 1 – 1.5% inter-storey drift (AS 1170.4)
A 1.5% drift for a 3600 mm storey height corresponds to a lateral deflection Δ = 54 mm. If
the capacity of the façade system is higher than this limit, then the façade is considered
safe and it is not necessary to carry out further assessment.
Tier # 2 – RSDmax from response spectrum (AS 1170.4)
The RSDmax method is suitable for buildings dominated by the first mode and typically
less than 10 storeys.
RSD max ×FPF ×FM ×FT
 1.5%
n×h
Where RSDmax = maximum displacement demand for site class
FPF = Participation factor (1.0 – 1.5)
FM = mode shape correction multiplier (1.0 - 2.0)
FT = Torsional amplification factor (1.0 - 2.0)
n = number of storey
h = storey height
Inter-storey drift =
(1)
The participation factor can be assumed FPF = 1.5 for buildings regular in elevation and
FPF = 1.0 for single storey buildings and soft-storey buildings. A value of Fm = 1 can be
used for building less than 5 storeys and conservatively the value of Fm = 2 can be used for
buildings between 5-10 storeys to account for the curved mode shape (Wilson and Lam
2006). The value of FT =1 is recommended for symmetric buildings, whilst values of FT =
1.6 and FT = 2.0 are conservatively assumed for estimating peak drift demands for
buildings that are asymmetric in one and two directions respectively (Lumantarna et al.
2008). Table 1a, 1b and 1c summarise the typical drift demands for different heights of
building for 500 year RP (Z = 0.1g) and 1500 year RP (Z = 0.15g ) events on different soil
types for regular, one and two directional asymmetric buildings in accordance with AS
1170.4 (2007).
No. of
Storeys
3
8
10
500
17
13
10
Class B
1500
25
19
15
Inter-storey drift (mm)
Class C
500
1500
24
18
14
36
27
21
Class D
500
1500
38
28
23
54
42
34
Table 1a Maximum drift demand on façade systems (regular buildings)
No. of
Storeys
3
8
10
Class B
500
1500
27
20
16
40
30
24
Inter-storey drift (mm)
Class C
500
1500
38
29
23
57
43
34
Class D
500
1500
54
45
36
81
68
54
Table 1b Maximum drift demand on façade systems (one directional asymmetric buildings)
No. of
Storeys
3
8
10
Class B
500
1500
34
25
20
50
38
30
Inter-storey drift (mm)
Class C
500
1500
48
36
29
71
53
43
Class D
500
1500
54
54
45
108
81
68
Table 1c Maximum drift demand on façade systems (two directional asymmetric buildings)
Tier # 3 – Δe from response spectrum (AS 1170.4)
The Δe method also is suitable for buildings dominated by the first mode and typically less
than 10 storeys. The natural period of the buildings is required. The effective moment of
inertia (Ie) should be used to represent the cracked sections rather than the gross sectional
properties (Ig). The use of Ig instead of Ie would potentially lead to a substantial
underestimate of the inter-storey drift (McBean 2008). Paulay and Priestley (1992)
recommended 0.40Ig for beams and 0.60Ig for normal columns as effective moments of
inertia. In this study an average 0.50Ig has been used for all elements to provide a
reasonable estimate of the building natural period. The typical displacement demand
corresponding to the effective stiffness (Δe) is shown in Figure 1.
Acceleration
T
Te
Δe
Displacement
Figure 1 Displacement for effective stiffness of a building
The inter-storey drift can be estimated from the Tier # 3 method as follows:
 e ×FPF ×FM ×FT
 1.5%
(2)
n×h
Where Δe = displacement demand corresponding to effective period (Te) and the remaining
factors are the same as explained in Tier # 2. Table 2 compares the typical drift demands
for the 500 year RP events with Z = 0.1g on different soil types for regular buildings (Ig
and 0.50Ig) in accordance with AS 1170.4 (2007).
Inter-storey drift =
No. of
Storeys
Period
T (s)
0.50Ig
period
Te (s)
3
5
8
10
0.3
0.5
0.8
1.0
0.43
0.71
1.13
1.41
Class B
Ig
0.50Ig
3.4
3.3
6.7
6.7
4.7
4.7
9.4
9.5
Inter-storey drift (mm)
Class C
Class D
Ig
0.50Ig
Ig
0.50Ig
4.2
4.7
9.5
9.5
6.7
6.7
13.4
13.4
4.2
7.0
15.1
15.0
8.4
10.6
21.3
21.3
Table 2 Maximum drift demand on façade systems (regular buildings)
Tier # 4 – Modal analysis using response spectrum method (AS 1170.4)
The method is suitable for buildings greater than 10 storeys where higher mode effects are
important. The soil class and modal properties including natural period of the building for
effective stiffness are required to undertake the analysis. The well known modal
combination rule, the square root of the sum of the squares method is used to calculate the
inter-storey drifts. The method is applied for three high-rise buildings. Table 3 summarises
the details of the buildings. Table 4 compares the typical drift demand on regular buildings
for 500 year RP event for soil classes in accordance with AS 1170.4 (2007).
Building
Reference and
location
1
Singapore
2
Melbourne
3
Singapore
Description
Frame-tube
Office block
Central core
steel frame
Office block
Concrete
Office block
Natural period (sec)
Height Number
(m)
of floors Mode 1 Mode 2 Mode 3
Reference
280
66
5.4
1.5
0.7
(Brownjohn and Pan
2001)
152
40
3.8
1.0
0.6
(Swaddiwudhipong et
al. 2002)
91
26
1.5
0.4
0.2
(Brownjohn and Pan
2001)
Table 3 Summary of the buildings
No. of
Storeys
Ig
26
40
66
Class B
0.50Ig
Inter-storey drift (mm)
Class C
Ig
0.50Ig
Ig
Class D
0.50Ig
2.7
3.0
3.8
4.2
5.9
6.2
2.8
3.2
3.9
4.5
6.2
7.2
1.7
2.0
2.4
2.9
3.8
4.5
Table 4 Maximum drift on buildings (Z = 0.1g and 500 YRP)
The time-history analysis approach of determining earthquake actions can result in more
efficient designs in comparison with using response spectra specified by the Standard
(Lam and Wilson 2005).
Tier # 5 – Capacity spectrum method
The capacity spectrum method can be used to estimate the likely inelastic displacement
demand of an equivalent single degree of freedom structure. The maximum drift demand
on the building can then be estimated from Equation 2 by substituting Δinelastic for Δe.
 inelastic ×FPF ×FM ×FT
 1.5%
(3)
n×h
The capacity spectrum method is illustrated using a real case study example involving a
soft-storey building. The building is illustrated in Figure 2a and has 5 storeys with the
ground floor open and much more flexible and laterally weaker than the upper storeys.
Consequently, under lateral loading the deformations are concentrated at the ground floor
level with the columns being forced to drift laterally whilst maintaining the gravity load of
the upper storeys. Wibowo et al (2008) undertook a unique pushover experimental study
of this building while measuring the lateral force versus displacement behaviour of the
building. This has been plotted on an ADRS format diagram as shown in Figure 2b, from
where the performance point can be estimated for 500 year RP event ( Z = 0.1g) for soil
classes. The inelastic displacement demand on a class ‘D’ site for this building is Δinelastic =
45 mm and the inter-storey drift can be calculated from Equation 3 assuming FPF = 1.0, FM
= 1.0 and n = 1.
Inter-storey drift =
Performance Point
Acceleration (m/s2)
4
AS1170.4-C
AS1170.4-D
Bay
3
2
1
0
0
(a)
0.05
0.1
0.15
Displacment (m)
0.2
0.25
(b)
Figure 2 (a) The tested soft storey building, (b) Pushover curve of the bays in weaker direction
4. IN-PLANE DRIFT CAPACITY OF GLAZED FAÇADE SYSTEMS
The glazed façades can be classified into two main types, namely framed glazed and
frameless glazed façade systems. The unitized curtain wall system is a more contemporary
framing method which comprises a glass vision panel and spandrel panel mounted in a
prefabricated aluminium frame and illustrated as a complete unit in Figure 3a.
Alternatively a new contemporary frameless glazed façade system is available which
provides transparency and improved aesthetics, known as point fixed or bolt fixed glazed
curtain wall system. Point fixed glazing systems are often connected with bolts to steel
support structures, (which are exposed architectural elements) to combine structural
stability with aesthetic expression. A typical bolted glazing system supported by trusses is
shown in Figure 3b.
(a)
(b)
Figure 3(a) Assembling of a stick curtain wall system and (b) Bolted glazing supported by truss system
Bouwkamp (1960) observed that in-plane deformation of window panels under lateral
loading takes place in two phases, as shown schematically in Figure 4. First, the window
frame deforms and the glass plate translates within the frame until contact occurs at two
opposite corners of the glass panel (Figure 4b). The glass panel then further rotates until
its opposite corners coincide with the adjacent frame corners (Figure 4c). Sucuoglu and
Vallabhan (1997) found that the total lateral deformation of the window panel due to rigid
body motion of the glass panel in the window frame can be expressed in terms of the
geometric properties of window panel components as:
 h
Δ = 2c 1+ 
 b
(4)
Where Δ is the lateral drift capacity of the glass frame and c, h and b are physical
dimensions as defined in Figure 4.
Figure 4 Movement of glass panel within window frame for a glazed window
(Sucuoglu and Vallabhan 1997)
Sucuoglu and Vallabhan (1997) suggested that, this expression is valid when the glass
panel is glazed with a soft sealant which permits the relative motion of glass panel with
respect to the window frame. As the sealant hardens due to ageing, the lateral drift
capacity of the widow panel reduces significantly. However, neoprene gaskets and other
soft sealants used in modern glazing systems possess sufficient resilience to accommodate
the relative motion of glass panels in window frames. Equation 4 indicates that the inplane drift capacity of the glazed frame, before glass breakage is only dependent on the
edge clearance and the aspect ratio. Typical edge clearances used in general practice range
from 6 mm to 13 mm. Table 5 compares the in-plane drift capacity of 3600 high frame
glazed curtain wall with different widths and edge clearances using Equation 3.
Height (h) Width (b) Aspect ratio
(mm)
(mm)
(h/b)
3600
3600
3600
3600
3000
2400
1800
1200
1.2
1.5
2.0
3.0
In-plane drift capacity for typical edge
clearances (mm)
c=6
c=8
c =10
c =12
26
35
44
53
30
40
50
60
36
48
60
72
48
64
80
96
Table 5 Typical in-plane drift capacity of framed glazed curtain walls
The in-plane drift capacity of a curtain wall with an aspect ratio of h/b = 2 and for the
typical minimum edge clearance of c = 6 mm is therefore Δ = 36 mm as summarised in
Table 5. This drift capacity satisfies the drift demand of buildings analysed in Tier # 3 and
Tier # 4. Some of the in-plane drift capacity figures in Table 5 also satisfy the demand of
54 mm as given in Tier # 1. It shows that for buildings with higher drift demands
(especially in soft-storey buildings) the in-plane drift capacity of the framed façades can
be modified by increasing the edge clearance or aspect ratio. Equation 4 can be modified
for uneven clearances between vertical and horizontal glass edges and the frame (ASCE 702 2002) as:

h p c2 
Δ = 2c1 1 +


b
c
p
1


(5)
Where hp = height of the rectangular glass panel, bp = width of the rectangular glass panel,
c1 = clearance (gap) between the vertical glass edges and the frame, and c2 = clearance
(gap) between the horizontal glass edges and the frame.
The seismic performance of point fixed (frameless) glazing is likely to be quite different
from conventional framed systems. McBean (2008) commented that the capacity of point
fixed glazing is at least half of the capacity of framed glazing. There is very limited
published research available on the behaviour of frameless glass façade systems under inplane actions earthquake loading and a reliable and rational testing program is required to
assess the drift performance of such systems (Sivanerupan S et al. 2008).
5. CONCLUSIONS AND SUMMARY
The seismic assessment of glazed façade systems requires an estimate of the likely drift
demand from the building. Codified and industry provisions for in-plane drift limits on
glazed façade systems are reviewed. Analysis results indicate that the inter-storey drift is
much less than the 1.5% limit in AS 1170.4 (2007) for most buildings in Australia for the
500 year RP event except for soft storey structures. It reveals that standard methods should
be used for estimating the in-plane seismic drift demands on glazed façade systems. A
tiered approach has been presented for estimating the seismic drift demand of glazed
façade systems with increasing levels of sophistication and accuracy. Applications of these
methods are illustrated with number of examples and conservative factors are presented
for considering the torsional behaviour of buildings.
The drift capacity of a framed glazed system is dependent on the edge clearance and the
aspect ratio before glass breakage. Results indicate that framed glazed façade systems with
typical minium edge clearance in regular buildings are not vulnerable except soft-storey
buildings for moderate earthquake events such as 500 year RP event. However for
torsionally unbalanced buildings, the maximum drift limit of 54 mm for a typical 3600
high floor can be achieved by increasing the edge clearance or aspect ratio of the glazed
frame.
Despite its growing popularity, there is very limited published research on the behaviour
of frameless glass façade systems under the in-plane earthquake loading. The seismic
performance of point fixed (frameless) glazing is likely to be quite different from
conventional framed systems. A reliable and rational testing and analytical work is
required to assess the drift performance of point fixed glazed façade systems. This is also
part of the on-going research undertaken by the authors to evaluate the vulnerability of
glazed façades under in-plane seismic loading.
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